1. Field of the Invention
The invention relates to a projection display, more particularly to a single-lens type projection display with a color separation/synthesizing prism unit.
2. Description of the Related Art
In a conventional projection display, white light is separated into first, second and third color components that are subsequently and respectively modulated by a corresponding light valve. The modulated color components are then synthesized to form an output beam prior to reception by a projection lens for projecting a color image on a display screen.
U.S. Pat. No. 5,644,432 discloses a three-prism color separator for use with three reflective light valves. As shown in FIG. 1, according to this patent, white light 10 from a polarizing cube 11 is separated into red, green and blue color components by an optical beam splitter 12 that includes first, second and third prisms 121, 122, 123. The three color components are directed by the three prisms 121, 122, 123 to three reflective light valves 131, 132, 133 for light modulation. Thereafter, the light-modulated color components retrace their paths through the three prisms 121, 122, 123 so as to be synthesized to form an output beam that is to be received by a projection lens (not shown) via the polarizing cube 11 for projecting a color image.
FIG. 2 illustrates another conventional projection display 2. The projection display 2 includes a light source 21 and a light filter 22 downstream of the light source 21 for removing ultra-violet rays and infra-red rays from a light beam generated by the light source 21 to obtain a white beam output. First and second beam splitters 231, 232 split the white beam output from the light filter 22 into first, second and third color components 201, 202, 203, such as red, green and blue color components. The first and second color components 201, 202 from the second beam splitter 232 are modulated by first and second transmissive light valves 251, 252, respectively. The third color component 203 from the first beam splitter 231 is reflected by a mirror 241 prior to modulation by a third transmissive light valve 253. After modulation, the three color components 201, 202, 203 are synthesized using two beam splitters 233, 234 and a mirror 242 prior to reception by a projection lens 26 for projecting an image on a display screen (not shown). Particularly, the first color component 201 is reflected once by the mirror 242 before passing through the beam splitter 233. The second color component 202 is reflected once by each of the beam splitters 234, 233. The third color component 203 passes through the beam splitter 234 before being reflected by the other beam splitter 233.
Because the three color components 201, 202, 203 either pass through or are reflected by the beam splitters 234, 233 and the mirror 242 in different ways during color synthesizing, minor differences inherent in the different optical paths can result in image distortion.
Therefore, the main object of the present invention is to provide a single-lens type projection display which utilizes a color separation/synthesizing prism unit to ensure uniformity of the optical path lengths for the different color components during color separation and color synthesizing to minimize the occurrence of image distortion.
Another object of the present invention is to provide a single-lens type projection display with a color separation/synthesizing prism unit and which has light modulators that use transmissive light valves.
According to this invention, a projection display comprises:
a light source for generating a beam output that contains first, second and third color components;
a color separation/synthesizing prism unit having a color separation part for separating the beam output from the light source into the first, second and third color components that exit the prism unit in three different directions;
first, second and third light modulator means, disposed adjacent to the prism unit, for modulating the first, second and third color components that exit from the color separation part of the prism unit;
the prism unit further having a color synthesizing part that receives the modulated first, second and third color components from the first, second and third light modulator means and that synthesizes the modulated first, second and third color components so as to form an output beam; and
a projection lens for receiving the output beam from the color synthesizing part of the prism unit, and for projecting a color image.
The first, second and third color components have equal optical path lengths measured from the light source to the respective one of the first, second and third light modulator means. The first, second and third color components further have equal optical path lengths measured from the respective one of the first, second and third light modulator means to the projection lens.
Preferably, the prism unit includes first, second and third prisms, and the first, second and third color components exit the color separation part of the prism unit at the first, second and third prisms, respectively.
In the preferred embodiment, the color synthesizing part is vertically disposed relative to the color separation part of the prism unit. Each of the first, second and third light modulator means includes a transmissive light valve, a first mirror for reflecting the respective one of the first, second and third color components from the color separation part of the prism unit vertically, and a second mirror spaced apart vertically from the first mirror. The light valve is disposed between the second mirror and the color synthesizing part of the prism unit. The second mirror reflects the respective one of the first, second and third color components from the first mirror to the light valve.